There's a tremendous variation in ease of fit among tubulars. Usually, but not always, lighter tires fit better. I've seen tires that fit so easily they seem too big, and others so short that I suspected they were for smaller rims.

Either way it doesn't matter because all tubulars are built with bias plies and work on the same principle as the Chinese finger puzzle so that pressure inside makes the tire fatter and shorter, constricting onto the rim. It's this bi-axial helix action which holds tires on, with the glue only providing traction so it cannot shift.